All patients with refractory epilepsy undergo anatomic imaging studies (MR or CT) prior to surgery in order to identify (or exclude) potential epileptogenic mass lesions (tumor or AVM). State-of-the-art T2-weighted MR imaging will identify close to 100% of all such mass lesions. However in our and other's experience, T2-weighted MR imaging typically will not provide definite preoperative seizure lateralizing information in most patients with temporal lobe epilepsy who are subsequently found histologically to have mesial sclerosis. Functional imaging studies (PET/SPECT) have been found to be more useful than T-2-weighted MR imaging for seizure lateralization in patients with mesial sclerosis. We have developed a technique of MR-based hippocampal formation volume measurement (HF) volumetry) which is both sensitive and highly specific in seizure lateralization in patients with mesial sclerosis. Therefore, the clinical role of our volumetric technique clearly overlaps the domain occupied by functional imaging (PET/SPECT). No data exists to indicate how these two vastly different approaches to the same problem (seizure lateralization by neuroimaging in patients without an epileptogenic mass lesion). We propose to evaluate HF volumetry in 180 patients who will undergo surgery at Mayo Medical Center for medically refractory partial epilepsy of temporal lobe origin over a 3 year period, who have pathologically documented mesial sclerosis in the surgical specimen. We hypothesize that I) unilateral hippocampal atrophy as determined by MR-based volume measurements, will be an accurate, non-invasive marker identifying the side of the seizure disorder, 2) the degree of unilateral hippocampal atrophy on MR will be useful in predicting the level of post surgical seizure control (surgical outcome). We also propose a comparative evaluation of 1) the percentage of cases in which the seizure focus is correctly and incorrectly identified by HF volumetry versus brain SPECT, 2) whether these imaging studies are, or are not complimentary in lateralizing the epileptogenic focus.